This invention relates in general to a fluid displacement device. More particularly, it relates to an improved scroll-type fluid displacement device which achieves a high built-in volume ratio without compromising other optimum design parameters. This invention also relates to a "semi-compliant" mechanism for maintaining the desired operative relationship between the scroll members of a scroll-type fluid displacement device.
Scroll-type fluid displacement devices are well-known in the art. For example, U.S. Pat. No. 801,182 to Creux discloses a scroll device including two scroll members each having a circular end plate and a spiroidal or involute scroll element. These scroll elements have identical spiral geometries and are interfit at an angular and radial offset to create a plurality of line contacts between their spiral curved surfaces. Thus, the interfit scroll elements seal off and define at least one pair of fluid pockets. By orbiting one scroll element relative to the other, the line contacts are shifted along the spiral curved surfaces, thereby changing the volume of the fluid pockets. This volume increases or decreases depending upon the direction of the scroll elements' relative orbital motion, and thus, the device may be used to compress or expand fluids.
Referring to FIGS. 1a-1d, the general operation of conventional scroll compressor will now be described. FIGS. 1a-1d schematically illustrate the relative movement of interfitting spiral-shaped scroll elements, 1 and 2, to compress a fluid. The scroll elements, 1 and 2, are angularly and radially offset and interfit with one another. FIG. 1a shows that the outer terminal end of each scroll element is in contact with the other scroll element, i.e., suction has just been completed, and a symmetrical pair of fluid pockets A1 and A2 have just been formed.
Each of FIGS. 1b-1d shows the position of the scroll elements at a particular drive shaft crank angle which is advanced from the angle shown in the preceding figure. As the crank angle advances, the fluid pockets, A1 and A2, shift angularly and radially towards the center of the interfitting scroll elements with the volume of each fluid pocket A1 and A2 being gradually reduced. Fluid pockets A1 and A2 merge together at the center portion A as the crank angle passes from the state shown in FIG. 1c to the state shown in FIG. 1d. The volume of the connected single pocket is further reduced by an additional drive shaft revolution. During the relative orbital motion of the scroll elements, outer spaces, which are shown as open in FIGS. 1b and 1d, change to form new sealed off fluid pockets in which the next volume of fluid to be compressed is enclosed (FIG. 1c and 1a show these states).
FIG. 2 diagrammatically illustrates the compression cycle that takes place in one of the fluid pockets, A1 or A2, as it converges toward the center portion A. FIG. 2 also illustrates the relationship between fluid pressure and volume in the fluid pocket.
The compression cycle begins (FIG. 1a) when the fluid pockets are sealed. In FIG. 1a, the suction phase has just finished. The fluid pressure in one of the fluid pockets in the suction phase is shown at point H in FIG. 2.
The volume of the pocket at point H is the displacement, V.sub.H. The volume of the fluid pocket is continuously reduced and the fluid is continuously compressed as the scroll element is rotated to a certain crank angle. This state is shown by point L in FIG. 2. The volume (V.sub.L) of the pocket at state L is defined as the final compression pocket volume. Immediately after passing point L, the fluid pockets, A1 and A2, are connected to one another and simultaneously connected to the central volume A which is filled with undischarged high pressure fluid.
The ratio of the suction pocket volume, V.sub.H, to the final compression pocket volume, V.sub.L, is defined as the built-in volume ratio, R.sub.V. The ratio of the pressure (P.sub.L) at state L to the pressure (P.sub.H) at state H is defined as the pressure ratio.
Referring back to FIG. 2, as the crank angle passes state L, the fluid in the connected fluid pockets, i.e. the central volume A, will undergo one of the following three processes:
1) Ideal compression: The ideal compression process occurs when the fluid pressure (P.sub.d1) in the central volume A, equals the pressure in the final compression pocket P.sub.L. The fluid discharges without pressure change as shown by the line L--L in FIG. 2. In this process the built-in volume ratio of the scroll members perfectly matches the operating condition, and hence, high energy efficiency is achieved in the compression process.
2) Overcompression: In this case, the fluid pressure (P.sub.L) in the final compression pocket at point n is higher than the pressure in the central volume P.sub.d2. As the crank angle passes point L, the fluid in the final compression pocket suddenly expands into the central volume and reduces its pressure until it equals P.sub.d2 as shown by point M in FIG. 2. The shadowed triangle LMO represents the energy loss due to overcompression.
3) Undercompression: In this case P.sub.L is lower than the discharge pressure P.sub.d3. As the crank angle passes point L, the fluid in the central volume rapidly expands into the final compression pockets, and the fluid pressure in the final compression pockets P.sub.L rises instantly to P.sub.d3 as shown by point N in FIG. 2. The fluid in the final compression pocket then discharges at line N--N. The shadowed triangle LNT represents the energy loss due to undercompression.
In order to achieve high energy efficiency, it is very important that the built-in volume ratio be designed as close to the ideal compression process as possible. Different applications require different built-in volume ratios to realize their respective ideal compression process. For example, a heat pump would require a ratio of about 4, an air compressor would require a ratio of about 5 and a low temperature refrigeration system would require a high ratio of about 10 or even higher. However, most conventional scroll devices cannot achieve these ratios. For example, in U.S. Pat. No. 3,884,599, the spiral elements of the scroll members span more than two but less than three full turns. Thus, the built-in volume ratio for this type of design is only about 2.5.
U.S. Pat. No. 4,477,238 discloses one method for achieving a high pressure ratio in a scroll-type displacement device by leaving the built-in volume ratio unchanged and placing a discharge valve, for instance, a reed valve, at the discharge port. Although this approach reduces energy loss, the valve is vulnerable to breakdown, and therefore, it increases the failure rate substantially. It also raises the noise level due to the vibration and impacting action of the valve.
Another approach to the problem is to increase the number of the turns in the spiral-shaped scroll elements. FIGS. 15 and 16 of U.S. Pat. No. 801,182 disclose one example of this approach. The scroll elements span approximately four full turns, and the built-in volume ratio can reach higher than three. Further increase in the number of turns, however, will increase machining costs and machining precision requirements. Increasing the number of turns may also be extremely impractical due to displacement requirements or space limitations.
The optimum number of turns for a scroll element is greater than two but less than three. With the optimum number of turns, the suction and discharge areas are always separated by at least one sealed off pocket. This is important in order to reduce the undesired leakage flow of both mass and heat between the two areas.
U.S. Pat. No. 3,989,422 discloses a method of constructing spiral-shaped scroll elements having a high built-in volume ratio and the optimum number of turns. According to this method, the first turn of the scroll element is designed in a conventional manner. In order to reduce the volume of the final compression pocket, and thus increase the built-in volume ratio, the scroll element suddenly and dramatically reduces its radius of curvature by moving the center of its generating circle toward one side. This method has serious shortcomings. As the central portion of the scroll element moves towards one side of its end plate, greater forces and moments are created due to the increased distance between the location where the compression forces act and the center of the end plate during its orbiting motion. To balance these forces and moments, the '422 patent provides a structure with multiple pairs of scroll elements in which the forces and moments cancel each other out. However, this structure increases machining time, machining precision requirements and material costs due to the complex structure and increased number of the scroll elements. Furthermore, the larger space requirements of the complex multiple scroll structure make it geometrically impractical to implement.
There are currently three approaches to maintaining an operative relationship between the scroll members in the "axial" direction (as measured linearly along the center axes of the scroll elements). These approaches may be referred to as "constant gap," "axially compliant," and "semi-compliant."
The constant gap approach was used in early devices as shown in U.S. Pat. No. 801,182 to Creux. In this approach, the relationship between the scroll members in the axial direction remains unchanged after the device is assembled. The tips of either scroll member do not contact the base of the opposing scroll member during normal operation. In order to maintain proper gaps between the scroll members and at the same time achieve high efficiency, extremely precise machining is required. Another more serious shortcoming of this approach is its inability to handle abnormal situations. If there are contaminants or incompressible fluid between the scroll members, or if the scroll members come into contact with each other due to excessive thermal growth, the scroll members could be damaged by galling.
To overcome the shortcomings of the constant gap approach, various types of axially compliant schemes have been developed. These schemes can be categorized as two types: "tip-seal" and "fully axially compliant."
The tip-seal scheme is shown in FIG. 10, and a further example is disclosed in U.S. Pat. No. 3,994,636 to McCullough et al. As illustrated in FIG. 10, a groove 501 is made in the middle of the tips of two scroll members, 502 and 503. A seal element 504 is loosely fitted in the groove 501 and urged by mechanical and/or hydraulic forces (not shown) into contact with the base 505 of the other scroll member, thus keeping fluid from leaking across the spiral scroll elements, 502 and 503, in the radial direction. However, the tip seal method inherently includes tangential leakage passages, as shown by lines A--A and B--B in FIG. 10, which reduce the compression efficiency. Other shortcomings of the tip seal method include friction power loss and the gradual deterioration of sealing effectiveness due to the seal elements wearing out.
In a fully axially compliant scheme, the scroll members maintain tip-base contact by mechanical or hydraulic forces, thereby sealing off the fluid pockets regardless of the pressure in the scroll device. U.S. Pat. No. 3,600,114 to Dvorak et al. discloses a scroll machine in which at least one of the scroll members is subject to axial forces, mechanical and/or hydraulic, to maintain two scroll members in sealed contact. In the '114 patent, fluid at discharge pressure is introduced to exert a bias force on the back of the end plate of scroll members. U.S. Pat. No. 3,884,599 to Young et al. discloses a fully axially compliant design in which the orbiting scroll is axially subject to a hydraulic urging force at the discharge pressure. U.S. Pat. No. 4,357,132 to Kousokabe discloses a scroll machine in which fluid at an intermediate pressure is used to urge the orbiting scroll member against the fixed scroll member. U.S. Pat. No. 4,216,661 to Tojo discloses a fully axially compliant scheme in which fluid external to the machine acts on the back of the orbiting scroll member to provide an axial bias. U.S. Pat. No. 4,611,975 to Blain discloses a fully axially compliant scheme in which an annular chamber formed at the interface of the scroll members is connected to a relatively low pressure source to "suck" the two scroll members together. U.S. Pat. No. 4,496,296 to Arai discloses a fully axially complaint scheme in which two pressure chambers are formed at the back of the orbiting scroll member. These pressure chambers are connected to the compression pockets at an intermediate pressure and to the central volume at the discharge pressure. This scheme maintains radial sealing of the scroll members over a wide operating range. U.S. Pat. Nos. 4,767,293 and 4,877,382, both to Caillat et al., disclose a fully axially compliant scheme in which a non-orbiting scroll member with resilient mounting means is urged toward the orbiting scroll member by gas at an intermediate and/or discharge pressure
The fully axially compliant schemes have several shortcomings. For example, the gas pressure used in these schemes is often derived from the compression pockets and/or the discharge chamber, and thus, may vary in accordance with changes in the operating conditions, i.e., the suction and discharge pressure. However, these changes are not always proportional to the separating forces acting on the tips and bases of the scroll members. Thus, as a design compromise, if the bias force is sufficient for a range of operating conditions about a particular point, it would not be enough to maintain stable operation at low suction pressure and low discharge pressure. On the other hand, the same bias force would be excessive for operating conditions at high suction pressure and high discharge pressure.
Another shortcoming of the fully axially compliant scheme is that the power loss due to friction between the contacting surfaces is not negligible. For operating conditions at high suction pressure and high discharge pressure, excessive hydraulic urging forces result in large friction power loss and serious wear, sometimes even causing damage due to tip-base galling.
Still another shortcoming of the fully axially compliant scheme is that the tip-base contact results in vibration and noise.
U.S. Pat. No. 4,958,993 to Fujio discloses a third approach to maintaining gaps between scroll members. This approach may be referred to as "semi-compliant" since the gaps between the scroll members in the axial direction may be enlarged by moving one scroll member away from the other.
The '993 patent teaches that the orbiting scroll member should be made movable in the axial direction, rather than the non-orbiting scroll member. This is done to keep the number of moving parts to a minimum since the orbiting scroll member is already movable and the non-orbiting scroll member is already stationary. Moving parts are a source of unwanted vibration and noise. Also, the orbiting scroll member is typically lighter than the non-orbiting scroll member, and thus the response time of the orbiting scroll member is quicker due to its smaller inertia.
There are several problems with the semi-compliant scheme taught by the '993 patent. For example, the potential for tipping the orbiting scroll member is greatly increased by making it movable in the axial direction. As seen in FIG. 3 of this application, the orbiting scroll member is subject to a driving force, F.sub.d, acting on the middle of driving pin boss 53, and to a reaction force, F.sub.g, from the compressed gas acting on the middle of the vane 51. These two forces are perpendicular to the axis, S1--S1, and form a moment which tends to tip the orbiting scroll member 50 and cause it to wobble as it orbits. The '993 patent teaches a range of movements (orbiting and axial) for the orbiting scroll member which makes it extremely difficult to balance the forces and moments acting on the scroll member and thereby prevent it from tipping. If the '993 parent's orbiting scroll member tips, it creates the same unwanted noise vibration and leakage that the '993 design was intended to avoid.
The present invention provides a new method of designing the scroll elements of a scroll-type fluid displacement device. Under the present invention, the design requirements for displacement, high built-in volume ratio and optimum number of turns are all satisfied. The present invention also provides an improved semi-compliant biasing scheme in which the potential for tipping is eliminated, thereby significantly reducing the amount of unwanted noise, vibration and leakage.